Recent wide computer use has entailed a popularity of information storage devices such as disc driving devices, including magnetic disc driving devices and the like. A disc driving device of a height of 2/3 inches or 1/2 inches, is prevailing among others in the market, because a handy type of computer in an A4 (letter) size employs this type of disc driving device. A spindle motor employed in this disc driving device is naturally required to be small, thin and light weight, because the storage devices are required to be the same. A bearing device supporting a rotational shaft of the motor must thus not only be small and thin, but also stable in rotation and strong against abrasion. It must also have a long life.
Japanese non-examined Patent Application Publication H07-243449 discloses a bearing device that aims to realize stable sliding and stable rotation of the rotational shaft. FIG. 5 is a cross sectional view of this prior art. In FIG. 5, a slide member 1 is cylinder shaped, and a cylindrical shaft-hole is punched through within a rim of the cylinder. The shaft-hole receives a rotational shaft (not shown) and supports it so that the shaft can be rotated in the circumferential direction. The shaft is thus supported rotatively within the slide member 1. The slide member 1 is press fitted into a holding member 2 and fixed thereto.
Diameters of the shaft-hole at a first end 51 and a second end 53 are smaller than that of a center part 52. Both ends 51 and 53 slide with respect to the shaft. A portion 54 of an outer rim of the slide member 1 corresponding to the end 51 of the shaft-hole has a smaller diameter than other parts of the slide member 1.
The slide member 1 is made of sintered metal that is impregnated with lubricating oil, i.e., it forms a sintered metal bearing device. This sintered metal bearing device splashes a little lubricating oil. In general, the life of this bearing device mainly depends on evaporation of the lubricating oil and abrasion due to operation. These factors are found as a function between temperature and time. Accordingly, the quantity of lubricating oil impregnated into the bearing device is a critical factor determining the life of this sintered metal bearing device. A predetermined quantity of the lubricating oil must be impregnated into the sintered metal in order to realize a long product life.
Because the structure of the above conventional bearing device has a limit to the amount of lubricating oil that can be impregnated thereinto, the cubic volume of the slide member 1 must be enlarged to secure the necessary quantity of the lubricating oil in order to ensure the desired life span. As a result, the entire body of the bearing device is obliged to be larger.
Another Japanese unexamined Patent Application Publication H08-9450 discloses a bearing device that prevents lubricating oil from splashing, and thereby prolongs the life of the bearing device.
FIG. 6 is a cross sectional view of this prior art. In FIG. 6, a porous member 62 is disposed axially near an end of a slide member 61 that supports a shaft 63. This porous member 62 prevents the lubricating oil from splashing out axially from a space between the shaft 63 and the slide member 61. This structure is effective to a certain extent to maintain a normal operation by preventing the lubricating oil from splashing; however, because the porous member 62 is disposed axially on end faces, the entire body of the bearing device is obliged to be axially longer. When a certain limit is set on the axial length, this bearing device has a disadvantage. If the axial length of the slide member 61 or the porous member 62 is shortened, the shaft-supporting length becomes shorter. As a result, the rotational accuracy of the shaft and the life of this bearing device are negatively affected.